176 SCIENCE PROGRESS. 



tained, the hydrocarbons and their halogen derivatives, the 

 ethers, the ethereal salts, and acid chlorides and anhydrides 

 are monomolecular liquids. Among- associating liquids one 

 must in the first rank place those compounds which contain 

 the hydroxyl group, as the presence of this group appears 

 to have the greatest influence in inducing association. 

 Thus water is found to occupy quite an exceptional position 

 among liquids, with a factor of association that is probably 

 higher than that of any other. The alcohols, and those 

 acids which contain the hydroxyl group, are also associating 

 liquids. The presence of certain other groups, among 

 which may perhaps be reckoned the amidogen and nitro- 

 groups, induces association, but probably in lesser degree 

 than does the hydroxyl group. 



In addition to the surface tension method, there are 

 several other means of judging whether a liquid is mono- 

 molecular or associating. None of these are, however, so 

 exact in principle as the method described, and the degree 

 of association can be only roughly gauged by these 

 processes. The chief methods are those based on an 

 examination of the liquid when it is undergoing a change 

 of state, either to gas or to solid, for it is fairly evident that 

 at these points the influence of association should be ren- 

 dered apparent in the most marked manner. 



Several methods of this kind have been indicated by 

 Guye, based chiefly on the behaviour of liquids either at or 

 near their critical points. Exceptions to Trouton's law, 

 that the molecular latent heat of vaporisation divided by 

 the boiling-point on the absolute scale is approximately 

 constant for all liquids, occur, as Linebarger has shown, 

 only with associating liquids. The molecular volumes of 

 monomolecular liquids are approximately proportional to 

 their boiling-points on the absolute scale, the value of 

 T Jmv being about 3*5 ; but in the case of associating liquids 

 this rule no longer holds and T/mv has a much higher 

 value (in the case of water 19/85). Combining this last 

 result with Trouton's law, it appears that the product of the 

 latent heat of vaporisation and the density at the boiling- 

 point (the latent heat of vaporisation of unit volume of the 



